Violent mergers of nearly equal-mass white dwarf as progenitors of subluminous Type Ia supernovae

Abstract

The origin of subluminous Type Ia supernovae (SNe Ia) has long eluded any explanation, as all Chandrasekhar-mass models have severe problems reproducing them. Recently, it has been proposed that violent mergers of two white dwarfs of 0.9 M_sun could lead to subluminous SNe Ia events that resemble 1991bg-like SNe~Ia. Here we investigate whether this scenario still works for mergers of two white dwarfs with a mass ratio smaller than one. We aim to determine the range of mass ratios for which a detonation still forms during the merger, as only those events will lead to a SN Ia. This range is an important ingredient for population synthesis and one decisive point to judge the viability of the scenario. In addition, we perform a resolution study of one of the models. Finally we discuss the connection between violent white dwarf mergers with a primary mass of 0.9 M_sun and 1991bg-like SNe Ia. The latest version of the smoothed particle hydrodynamics code Gadget3 is used to evolve binary systems with different mass ratios until they merge. We analyze the result and look for hot spots in which detonations can form. We show that mergers of two white dwarfs with a primary white dwarf mass of ~0.9 M_sun and a mass ratio larger than about $0.8$ robustly reach the conditions we require to ignite a detonation and thus produce thermonuclear explosions during the merger itself. We also find that while our simulations do not yet completely resolve the hot spots, increasing the resolution leads to conditions that are even more likely to ignite detonations. (abridged)